Cathode ray tube



Sept. 12, 1939. LEVlN 2,172,739

. CATHODE RAY TUBE Filed Feb. 2; 1957 Fig. 1

INVENTOR NYMAN L VIN All/W ATTORNEY Patented Sept. 12, 1939 2 1'ZZ 73 UNTD STATES PATENT if? 'OATHODE RAY TUBE v Nyman Levin, Finchley, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application February 2, 1937, Serial No. 123,564

In Great Britain February 3, 1936 5 Claims. (01. 250-162) This invention relates to cathode ray tubes and picture and the number of scanning lines. For has for its object to provide improved cathode example, if the spot on the screen is to be 0.02" ray tubes suitable for use in television systems square, or if it is to be a circular spot of .02" and for similar oscillograph purposes where moddiame t e arrangement y b SO Operated 5 ulation of the intensity of a scanning cathode ray that a spot 01 this size is obtained when the con- 6 beam is required. trol electrode is at cathode potential. According- In present day cathode ray television repro ly when the control electrode becomes negative ducer tubes it is usual practice to focus a reduced W th espect to the cathode the spot will become image of the cathode on th flu r c t screen smaller and of reduced intensity, but it can never 10 in the tube but a defect of this practice is that me lar er than the predet rmined size. 10 modulation of the intensity of the cathode ray The p nyin Fi ure 1 illus r es one form produces alteration in the size of the spot on the of construction in accordance with the invention. screen and in general a halo is introduced In Figure 1 a cathode r y u comprises W round the image when the control electrode of in an evacuated envelope E a cathode i; an aperthe electron gun is at zero or negative potential. tur'ed control electrode 2 which is very close to 15 It has been proposed to cause the first anode of said cathode and has an aperture conside ab the electron gun to form th aperture wh smaller than the cathode area; an accelerating image is focused on the screen, but known proanode 3 which is at a relatively large distance posals of this nature present the disadvantage f m he on r l l rode (as mp r to h of increasing the number of electrodes necessary control electrode-cathode spacing) and has also a 20 since the first anode does not take part in the small aperture; a relatively long cylindrical elecactual focusing. trode l having a small central aperture in each In accordance with one feature of this invenof i s end f ces; a a r h r p r ur d an tion, the control electrode of the electron gun of 5. These electrodes constitute the electron gun a cathode ray tube is placed very close to a cathsystem and co-operate to project a ray of elec- 25 ode whose dimensions are large relative to the size trons towards a screen 6 which is mounted near of the aperture in the said control electrode and the other end of the tube and may, if desired, confocusing is then accomplished upon the said aperstitute also a final anode. In practice, if the ture by a succeeding electrode assembly constitutcathode i be taken as of zero pot the 80 ing an electron lens assembly and so arranged as trol electrode 2 may be varied from -20 volts to be substantially unaffected by varying voltto zero, the c l r nod 3 y be m inages applied to the control electrode. This intained at about 2,000 volts the cylindrical elecvolves that the first anode must be at a relativ l trode i (hereinafter referred to for convenience large distance from the control electrode (as of description as the Second anode) y be 36. compared to the control electrode-cathode spacmaintained at zero potential, and the further ing) and must have a relatively much higher poanode 5 may be maintained at about 3,000 volts. tential. For example, if the distance between the The three anodes 3, 4, 5, may be a a t0 D control electrode and the cathode is .005 then duce an image of the aperture in the control the distance between the first anode and the conelectrode 2 on the screen 6 when the said control 4|) trol electrode should be about .030" and the poelectrode is at about 10 volts, and if this be tential of the said first anode should be of the done a spot of substantially unvarying size will order of 2,000 volts. The electrode assembly sucbe obtained. Alternatively the three anodes 3, 4, ceeding the first anode may be designed to give 5 ay be arr to focus an image f the p either an electron accelerating or an electron deture of the control electrode on the screen when celerating region as may be required. The spot the control electrode potential is zero, and if this 45 obtained on the screen will be a small image of be done the spot, although varying in size, will the aperture in the control electrode and the arnever exceed a p edete n tude Wh ch rangement may be so operated that this image it has for zero control electrode potential. will change substantially only in intensity and In accordance with another feature of the innot in size or shape when the potential on the vention an electronic field lens is employed to 50 control electrode is changed by voltage moduobtain good defin 0f the p in a a od lated signals. Alternatively the arrangement ray tube, and this lens is situated at a prelimimay be so operated that the spot on the screen, nary focus of a cathode or aperture which it is although changing in size, never increases bedesired to focus on the screen. This feature of beyond a dimension determined by the size of the the invention enables satisfactory focusing to be obtained and substantially avoids aberration and other forms of distortion without any need to introduce stops.

It will be appreciated that if focusing is obtained with a system including an electronic field lens situated at a preliminary focus of the cathode or aperture which it is desired to focus on the screen, and if the image is exactly in the plane of this lens, the size or focus of the image will not be afiected. Any halo around the image must be diverging from. a point outside the image plane and hence is caused to diverge from the field lens at so wide an angle as to be received by the plate or cylinder of the succeeding focusing lens. Thus the proportion of the halo passing through the aperture of the focusing lens will be so reduced as to be negligible.

The accompanying Figure 2 shows a construction embodying the last described feature of this invention. evacuated envelope a cathode I succeeded by a relatively closely spaced control electrode 2 whose aperture is smaller than the effective area 01 the cathode. The control electrode is in turn succeeded by three anodes 3, 4, 5, in succession, one (3) being an apertured plate, the second s) being a relatively long cylindrical electrode hav ing a central aperture in each of its end faces, and the third (5) being also an apertured plate. The tube further contains a screen 0 which, in this construction, is also an anode. Suppose it be desired to focus an image of the control electrode upon the screen. Then according to the potentials of the electrodes there will be formed preliminary focus F! in or near the aperture of that end plate of the cylindrical electrode l which is nearest the screen. Any halo will be caused by beam focusing at one side or other of the focus Fl, and it will be appreciated that such rays as come to a focus at these longitudinally displaced points will, in passing through the cylindrical anode 4 (which provides the field lens action) be caused to become divergent through such a wide angle that they will be stopped by the anode 5, i. e., they will miss the aperture in the anode 5 altogether and will strike the metal of' the said anode. In this way halo forming rays will be stopped but the correct desired beam which forms the spot image will-not be aiiected. This action is illustrated in the explanatory diagrammatic accompanying Figure 3, in which the chain lines represent halo-forming rays which pass through longitudinally displaced preliminary foci and will be stopped by the anode 5. In Figure 2 the chain lines also represent rays, but these rays are correctly focused rays which pass through the aperture in the anode 5.

The accompanying Figure 4 illustrates a third construction in accordance with this invention and embodying both features of the said invention. Here the tube comprises within an evacuated envelope a cathode l a control electrode 2 in the form of an apertured plate closely adjacent thereto, a first and second anode 3 and So. each in the form of an apertured plate, a third anode 4 in the form of a cylindrical electrode having a central aperture in each of its end faces, and a fourth anode 5 in the form of an apertured plate in a cylinder do. All the apertures are, of course, in alignment from the cathode. The cylinder 5a may form part of a metal screening deposit provided on the interior wall of the envelope the screening being continued as shown at 5?) into the usual flared part of the envelope. An example of practical electrode spacing for the Here there is employed within an.

construction of Figure 4 is: distance between the control electrode 2 and the first anode 3 approximately .020"; distance between the first and second anodes 3, 3a about .020"; distance between the cathode l and the electrode 2--as small as possible practically; distance between the first aperture in the cylindrical electrode 4 (this aperture constitutes the field lens) and the anode 3a about 0.2". The first and second anodes 3, 3a form an image of the aperture of the control electrode 2 on the first aperture in the cylindrical electrode 4 and the second aperture in the said cylindrical electrode 6, in conjunction with the fourth anode 5, re-focus this image on the normally provided screen (not shown) at the end of the tube. For the practical case above cited the length of the cylindrical electrode 4 may be approximately l" and the distance between the second aperture in this electrode and the aperture in the fourth anode 5 may also be about 1 and satisfactory results Will be obtained with potentials of 1,000 volts, zero, 3,000 volts and 15,000 volts applied to the first, second, third and iourth anodes respectively. As regards aperture sizes the control electrodes 2 and the first anode 3 may, for the case cited, each have an aperture of .025" and the aperture in the second anode may be .035", the first and second apertures in the third anode being 0.50 and 0.2" in diameter respectively and the aperture in the fourth anode being 0.25 in diameter.

Having described my invention, what I claim is:

1. .A cathode ray tube comprising, in the order stated, a cathode having a predetermined efiective area of electron emission, an apertured disk control electrode closely adjacent thereto, the area of the aperture of said disk being smaller than the predetermined efiective area of the cathode, two apertured disk anodes adjacent the control electrode, a cylindrical electrode having apertured end faces, and an apertured tubular electrode whose outside diameter is larger than the diameter of the cylindrical electrode, all of the apertures being in alignment along the axis of the cylindrical electrode and all of the apertured members being parallel to one another and lying in planes perpendicular to said axis.

2. A cathode ray tube comprising in the order stated a cathode having a predetermined eliective area of electron emission, an apertured flat disk control electrode closely adjacent thereto, the area of the aperture of said disk being smaller than the predetermined effective area of the oathode, an apertured fiat disk anode immediately adjacent the control electrode, a cylindrical electrode having apertured end faces, and a further apertured fiat disk electrode, all of said apertures being aligned along a line passing through the axis of the cylindrical electrode and all the flat disk electrodes and anode being parallel to one another and lying in planes perpendicular to said line.

3. A cathode ray tube comprising in the order stated a cathode having a predetermined efiective area of electron emission, an apertured fiat disk control electrode closely adjacent thereto, the area of the aperture of said disk being smaller than the predetermined efiective area of the cathode, two apertured flat disk anodes immediately adjacent the control electrode, a cylindrical electrode having apertured end faces, and a further apertured fiat disk electrode, all of said apertures being aligned along a line passing through the axis of the cylindrical electrode and allel to one another and lying in planes perpendicular to said line.

4. A cathode ray tube comprising in the order stated a cathode of relatively large emitting area, an apertured fiat disk control electrode closely adjacent to and in register with the cathode, the area of the aperture of said control electrode being smaller than the emitting area of the cathode, a first apertured flat disk anode positioned in register with and at a relatively large distance from the cathode compared to the distance between the control electrode and the cathode, a tubular electrode having apertured end faces, and a second apertured anode, said tubular electrode and second anode being in register with said cathode.

5. A cathode ray tube comprising in the order stated a cathode having a predetermined effective area of electron emission, an apertured flat disk control electrode closely adjacent thereto; the area of the aperture of said disk being smaller than the predetermined efiective area of the cathode, two apertured fiat disk anodes immediately adjacent the control electrode, a cylindrical electrode having apertured end faces, and an apertured tubular electrode, all of said apertures being aligned along a line passing through the axis of the cylindrical electrode and all the flat disk electrodes and anodes being parallel to one another and lying in planes perpendicular to said line.

NYMAN LEVIN. 

